This invention relates to a total wrist prosthesis designed for surgical replacement of the wrist joints in patients such as those suffering from rheumatoid degenerative and traumatic arthritis, and similar conditions.
Surgical replacements of human joints have been successfully achieved in recent years, particularly for the finger and hip joints. Although there is a recognized need for a total wrist joint replacement, no device fully effective for this purpose has been available. The few known wrist prostheses are basically simple ball and socket structures which provide unconstrained freedom of movement in all planes which intersect the center of the ball. Clinical trials have shown that such freedom of movement may lead to a permanently deformed position of the wrist which substantially limits its motion.
One existing prosthesis, called a Meuli prosthesis, includes a three-part ball and socket and is provided with intramedullary stems for methylmethacrylate fixation in the radius and metacarpal bones. The distal component is provided with a socket and intramedullary stems for the second and third metacarpals. A ball located between the metacarpal stems is manufactured either with or without a neck. Mechanically, the Meuli prosthesis provides equal ranges of motion for both radial-ulnar deviation and flexion-extension motion. The ball and socket design permits unwanted axial rotation when torque is applied to the hand around the forearm axis. The stability of the Meuli prosthesis depends entirely on the tendons and surrounding soft tissues, with the wrist tendons being relied upon to apply a proper tension to maintain the prosthesis at a suitable resting position. Bending of the stems of this prosthesis is generally required prior to implantation to offset the center of rotation from the axis of the radial stem and radius. The absence of such an offset is considered undesirable according to the article "Total Wrist Arthroplasty: A Preliminary Report." The Journal of Hand Surgery, Vol. 2, No. 5, 1977, pp. 337-344, by R. D. Beckenbaugh and R. L. Linscheid.
Another wrist prosthesis is the Loda prosthesis made in Argentina. It has a distal component, a cylindrical socket component with a spherical ball-receiving recess, and a proximal metallic component which includes a stem received in the intramedullary canal of the radius. The distal component includes a single intramedullary pin and is provided with antirotary flaps which prevent its rotation relative to the metacarpal bone. The socket component is affixed to the proximal component at a location which offsets the center of rotation from the axis of the radial stem. As in the Meuli prosthesis, the range of motion of the Loda prosthesis is the same in all directions and the prosthesis permits rotation of the distal component about the forearm axis if torque is applied to the hand. The Loda prosthesis has more stability but less range of motion than the Meuli prosthesis. It also has more constraint in the ball and socket joint. However, laboratory tests have shown that at extreme hand positions, the rim of the cylindrical socket acts as a fulcrum which can lead to dislocation of the ball and socket when a small load is applied at the end of the hand.
A third wrist prosthesis, known as the Volz prosthesis, is an unconstrained two piece device formed of a proximal component and a distal component formed respectively of ultra-high molecular weight polyethylene and Vitallium alloy. Its reported ranges of motion are 90.degree. in flexion-extension movement and 50.degree. in radial-ulnar deviation. A 1976 report on this prosthesis described seventeen cases of clinical results. This prosthesis has not been tested by the present inventors, but its description suggests that its center of rotation is aligned with the radius stem axis. This is undesirable as previously mentioned. Also, due to the unconstrained joint construction of the Volz prosthesis, it has little stability.
The present invention is believed to surpass prior wrist prostheses in its resemblance to the characteristics of a normal human wrist. The normal wrist joint, formed of eight carpal bones arranged in a distal row and a proximal row, is dynamically balanced by muscle forces, ligament forces, bony contact forces, and the viscoelastic forces of the surrounding soft tissue. It is a biaxial joint with two degrees of freedom which provide flexion-extension motion and radial-ulnar deviation. Flexion-extension motion occurs at the intercarpal and radiocarpal joints simultaneously, the range of such motion being approximately 80.degree. from neutral to flexion and 70.degree. from neutral to extension. In radial-ulnar deviation, the neutral-to-radial motion occurs mainly between the distal and proximal carpal rows, the proximal row of carpal bones remaining stationary. During neutral-to-ulnar deviation, there are intercarpal and radiocarpal joint motions which cumulatively provide an ulnar deviation which is larger than the radial deviation.
The center of normal radial-ulnar rotation in a dorsal or PA x-ray view is offset from the forearm axis towards the radius one-twelfth of the total width of the radius and ulna. It is also offset ulnaward from the radius axis a distance which is about one-tenth of the length of the third metacarpal. The total width of the radius and ulna is the distance from the ulnar border of the ulna to the radial border of the radius, measured perpendicular to the ulna axis at any point in the distal one-third of the radial and ulnar diaphyses. The center of flexion-extension rotation, seen in a lateral x-ray view, is displaced volarward from the radius axis a distance which is about 10% of the third metacarpal length.
The prosthesis of this invention includes a proximal component and a distal component, which are provided with means for connecting them respectively to the forearm and metacarpal bones. Means are provided for connecting the proximal component to the distal component to prevent axial rotation between the components relative to the forearm axis and to provide pivotal movement of the distal component relative to the proximal component, such movement being about two substantially perpendicular axes which are transverse to the forearm axis for radial-ulnar hand movement and flexion-extension hand movement. The pivot axis for radial-ulnar hand movement is preferably located at a position laterally offset ulnaward from the radius stem axis when the connector means is implanted in the lower forearm. Preferably, the distal component is connected to the metacarpal bones by means of a pair of diverging intramedullary stems which are stationary with respect to each other for engagement in the second and third metacarpal bones.